已知一正方体，各边长a，沿对角线BH作用一个力F，则该力在x1轴上的投影为____________。http://image.zhihuishu.com/zhs/onlineexam/ueditor/201812/b0e0cae96b734bf3a3dfbc78eee28790.png

力 作用在 OABC 平面内， x 轴与 OABC 平面成 q 角（ ），则力 F 对三轴之矩为 __ ___________ 。http://image.zhihuishu.com/zhs/onlineexam/ueditor/201812/b56e7eb2adca4d6b8689f328d472fdbc.png

已知一正方体，各边长a，沿对角线BH作用一个力，则该力在z1轴上的投影为___________。http://image.zhihuishu.com/zhs/onlineexam/ueditor/201812/5e81faaf5b7f42e790a3822d7f884c75.png

下列化合物有旋光性的是( )。 A: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201907/42754375451947b4a2097a204914e285.png" border="0" vspace="0" /> B: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201907/259cc0f9369441888e4306c19039152f.png" border="0" vspace="0" /> C: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201907/c6798f5eb7e14df7a79f40cdd7d2f42e.png" border="0" vspace="0" /> D: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201907/4275928f484d4bad94705297fe159cf0.png" border="0" vspace="0" />

空心圆轴的外径为D，内径为d, α=d/D。其抗扭截面系数为( )。 A: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201907/a4529f25a3514c4fb3fb60740f65c915.png" /> B: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201907/ed201b86c65c4e91be69e526278e5396.png" /> C: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201907/997a80bcc1dd4cbd9aacbdc8724ccaf9.png" /> D: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201907/c3e0ca0958684b89bce9d01917a38b5a.png" />